The lettuce aphid (Nasonovia ribisnigri (Mosley) is a major pest occurring in lettuce worldwide. The problem started to be severe for lettuce production in the 1970's in North Western Europe and spread rapidly all across Europe. In the 1980's, the aphid was detected in Canada. Later on, the problem was reported in the USA (California and Arizona). More recently, the lettuce aphid was found in New Zealand and Australia.
Lettuce aphids can colonize lettuce plants at any plant stage and feed preferably from younger leaves. Large amount of aphids on the plant are able to reduce plant growth and deform the shape of the head so that the lettuce heads are then not marketable. The presence of high amount of aphids in lettuce heads is a reason for retailers to refuse to buy lettuce from growers. At young plant stage, it is possible to control the lettuce aphid using insecticide. Several products were reported efficient to control aphid population. However, resistance to chemicals were reported in some aphid population. Moreover, at maturity, it is not possible to control aphid using insecticide because the chemical product cannot enter the lettuce head.
One of the most valuable strategies to control lettuce aphids is the genetic resistance. Extensive genebank screening was performed and some Lactuca virosa accessions were found completely resistant to Nasonovia ribisnigri. However, Lactuca virosa is in the second gene pool of the Lactuca germplasm according to the definition of Harlan. Therefore, these interspecific crosses are sterile, and the use of bridge species (as Lactuca serriola) was necessary to transfer the resistance into L. sativa. Genetic analyses showed that the resistance to Nasonovia ribisnigri was controlled by a single dominant gene (Nr gene) in a Lactuca sativa background.
However, breeders experienced that the release of varieties resistant to lettuce aphid was not straightforward. The Nr-resistance gene was found tightly linked to recessive genes conferring strong pleiotropic effect. Such plants showed a reduced growth, a pale green colour and a lack of fertility in seed set. Using large-sized progeny and molecular markers enabled Rijk Zwaan lettuce breeders to find resistant recombinant plants without the negative side-effect phenotype.
After this finding, the release of varieties resistant to Nasonovia became more and more important. The resistance trait became a major requirement for outdoor lettuce production for the processing industry and also for the fresh market. More recently, partial resistance to Nasonovia was reported in one L. serriola accession PI 491093 (Mc Creight, 2008)
In 2007, populations of lettuce aphids able to infect varieties resistant to Nasonovia ribisnigri were found in 4 distinct areas in Europe (France, Germany, Belgium and Austria). Four isolates (2 from France and 2 from Germany) were analysed further by Naktuinbouw (Netherlands Inspection Service for Horticulture) and they concluded the existence of a new Nasonovia ribisnigri biotype. This biotype is officially named Nr:1 and is able to overcome the Nr resistance gene. Nr:0 biotype can still be efficiently controlled by the Nr gene. Naktuinbouw also reported that IVT 280, the original source of the Nr:0 resistance, was susceptible in their tests.
After the breakthrough of the Nr gene by the Nr:1 biotype in practice, there was a need to find a resistance to the Nr:1 biotype.
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